Wellbores drilled into subterranean formations may enable recovery of desirable fluids (e.g., hydrocarbons) using a number of different techniques. Knowing the location of a target wellbore may be important while drilling a second wellbore. For example, in the case of a target wellbore that may be blown out, the target wellbore may need to be intersected precisely by the second (or relief) wellbore in order to stop the blow out. Another application may be where a second wellbore may need to be drilled parallel to the target wellbore, for example, in a steam-assisted gravity drainage (“SAGD”) operation, wherein the second wellbore may be an injection wellbore while the target wellbore may be a production wellbore. Yet another application may be where knowledge of the target wellbore's location may be needed to avoid collision during drilling of the second wellbore.
Electromagnetic ranging tools may be employed in subterranean operations to determine direction and distance between two wellbores. Electromagnetic ranging tools may use different techniques to obtain current on a conductive member (e.g., a casing string) in the target wellbore. Approaches may include directly injecting a current into the conductive member and/or inducing a current on a conductive member by transmitting electromagnetic fields by coil antennas positioned in a second wellbore. The induced current in turn may cause the casing to radiate a secondary electromagnetic field. In another approach, an electrode type source may be used to induce current on the conductive member. The gradient of the magnetic field radiated by the conductive member in addition to the magnetic field itself may be measured. Using a relationship between the magnetic field and its gradient, a ranging measurement may be calculated. Alternatively, an inversion may be used to determine the range, in which a forward model of the signal that may be received at the ranging tool may be needed. The inversion process may try to find the formation and well parameters that would match the forward model with the measurements made by the tool.